The present invention relates to a parallel rule, and in particular relates to such a parallel rule having an improved rolling device to reduce the thickness of the rule body and adapted for facilitating drawing.
The parallel rule is an essential tool for a draftsman. Conventional parallel rules are generally constructed of a rule body 44, a plurality of positioning means 41 and 42 and a leading line 43, as shown in FIG. 6. By means of the leading line 43, the rule body is movable to draw horizontal lines. Generally speaking, the base of the rule body 44 contacts the drawing-paper so as to smear the ink or lead on the paper when the ink is not completely dry.
Therefore, those skilled in this art have found that a protrusion 51 slightly projecting from each distal end of the rule body 5 facilitates the movement of the rule body, as shown in FIG. 5. Unfortunately, such a construction is easily deformable after a long period of use. Further to this, another improved parallel rule, as shown in FIG. 4, has a plurality of beads 62 disposed under the rule body 6 to enable the rule body 6 to move conveniently. However, such a parallel rule also creates friction points during moving so as to smear ink or lead the paper. Moreover, the beads 62 make much noise when rolling.
Furthermore, another improved parallel rule is constructed as shown in FIG. 3, wherein the rule body 7 has a rolling mechanism 71 and a shaft 72 located therein for loading a roller 73 which is slightly projectable from the bottom of the rule body 7. By means of the roller 73, the rule body 7 is movable.
Such a construction widens the required thickness of the rule body 7 so as to maintain the rolling mechanism therein. Also, the construction of this type of parallel rule is rather complicated.